Conventional free piston Stirling machines have a displacer piston and a power piston which reciprocate in a cylinder formed within a housing. At one end of the power piston there is a work space and at the opposite end, a back space. The displacer piston reciprocates in the work space and is connected to a spring apparatus which applies a force to the displacer piston. In conventional free piston Stirling machines, gas or mechanical springs are connected to the displacer piston either by a rod which slides through the power piston or by being formed within the displacer and acted upon by a rod fixed to the housing and extending through the piston.
A single chamber gas spring is the most commonly used spring apparatus. While fulfilling the basic spring function of periodically storing as potential energy the energy transferred to it by the kinetic energy of the moving displacer, the single chamber gas spring has a number of disadvantages. With the single chamber gas spring, hysteresis losses can be significant if high spring rates, that is large pressure variations, are required (such as in a high frequency, low pressure Stirling engine). Furthermore the seal length, the length of the region where the gas spring piston slides in the gas spring chamber walls, varies over the path of reciprocation of the gas spring piston. This variation in seal length could lead to unequal volume gas leaks into and out of the spring chamber, thereby causing the displacer piston to creep from its center position.
A graph of working gas pressure versus working gas volume from a machine using the single chamber gas spring is generally elliptical. The area within the graph (the ellipse) represents the amount of work produced by the machine. Since it is desirable to increase the amount of work a machine produces, it is desirable to increase the area within the graph. With the single chamber gas spring, the area within the graph is limited to being generally symmetric, such as an ellipse, because of the symmetric behavior of the gas spring. By the symmetric behavior of the spring it is meant that the force exerted by the spring when the displacer is at any point in the displacer's path, is similar to the force exerted by the spring when the displacer is at a point an equal distance from the center position of the displacer, but on the opposite side of the center.
While the conventional, single chamber gas spring has found the widest use, its many disadvantages demonstrate the need for an improved gas spring apparatus. Therefore, the need is created for a gas spring apparatus which shows low hysteresis losses, has a constant seal length to prevent piston creep and increases the amount of work produced by the Stirling engine.